2640
Znorg. Chem. 1993, 32, 2640-2643
An Unusually Weak Intervalence Transition in a Very Stable Bis-Chelate Analogue of the Mixed-Valent Creutz-Taube Ion. UV/Vis/Near-IR and EPR Spectroelectrochemistry of [(NH3)4Ru(p-bptz)Ru(NH3)41”’ (bptz = 3,6-Bis(2-pyridyl)-l,2,4,5-tetrazine;n = 3-5) Jiirgen Poppe, Michael Moscherosch, and Wolfgang Kaim’ Institut fiir Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-7000 Stuttgart 80, Germany Received November 11, 1992
The pentacationic form 25+ of the title complex, which exhibits an electrochemical stability constant Kc of 1015 in acetonitrile, shows much formal similarity to the mixed-valent (RuI1/RuIt1)pyrazine-bridgedCreutz-Taube ion 15+. However, the flanking of the central *-accepting tetrazine ring of the bridging ligand bptz by two coordinating pyridyl groups results in a rigid chelate arrangement with the ruthenium-ammine bonds situated parallel and perpendicular to the r system of bptz. This conformation is essentially different from the “staggered”arrangement in 15+,the difference being reflected not by the shape or maximum of the band due to the intervalence transition (IT) at 1450 nm but by a decrease of its intensity by 1 order of magnitude from e 5000 (15+)to 500 M-l cm-1 for P+.While the IT band maximum is essentially solvent insensitive, the near-IR spectrum in DzO/DCl reveals some structuring of the IT band with a spacing of about 900 cm-l. In addition to UV/vis/near-IR spectroelectrochemistry, the EPR data confirm the metal-based spin in the 5+ form (gl = 2.019, g2 = 2.418, g3 = 2.913) and a predominantly ligand-centeredunpaired electron in the 3+ state (gll = 2.022, g, = 1.989),which is accessible here due to the good r-acceptor propertiesof bptz. Nevertheless, the observability of the EPR signal only below 70 K indicates a stronger metal contribution in Z3+ than in the related radical complex [ (bpy)2Ru(pbptz)Ru(bpy)2]3+.
Experimental studiesin the field of mixed-valence coordination chemistry’ have been dominated for some time by ruthenium(II/III) ammine complexes,lJ most of them modeled after the In addition to the pyrazine-bridged Creutz-Taube ion 15+.2.3 question of valence localization or delocalization, much of the research has focused on the understanding of spectral properties, especially of the characteristic long-wavelength metal-to-metal charge-transfer (MMCT) or intervalence transitions on the basis of theoretical models.5 Within our attempts to broaden the experimentalbasis of mixed-valencechemistryby introducing new bridging ligands6 and organometallic complex fragments’ related to [Ru(NH3)sl2+/3+,we have now studied the optical and EPR spectral properties of odd-electron states of [p3,6-bis(2pyridy1)-1,2,4,5-tetrazine]bis(pentaammineruthenium) (2”+). (1) (a) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967,10,247.
(b) Brown, D. M., Ed. Mixed-Valence Compounds;Reidel: Dordrecht, The Netherlands, 1980. (c) Prassides, K., Ed. Mixed Valency Systems-Applications in Chemistry, Physics and Biology; Kluwer: Dordrecht, The Netherlands, 1991. (2) (a) Creutz, C. Prog. Inorg. Chem. 1983,30, 1. (b) Richardson, D. E.; Taube, H. J . Am. Chem. SOC.1983, 105, 40. (c) Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984,60,107. (d) Tanner, M.; Ludi, A. Inorg. Chem. 1981, 20, 2348. (3) Creutz, C.; Taube, H.J . Am. Chem. Soc. 1969, 91, 3988; 1973, 95, 1086. (4) Blasse, G. Struct. Bonding (Berlin) 1991, 76, 153. (5) (a) Schatz, P. N. In ref IC, p 7. (b) Neuenschwander, K.; Piepho, S. B.;Schatz,P. N. J . Am. Chem.Soc. 1985,107,7862. (c) Zhang, L.-T.; KO, J.; Ondrechen, M. J. J . Am. Chem. SOC.1987, 109, 1666. (d) Ondrechen, M. J.; KO, J.; Zhang, L.-T. J . Am. Chem. Soc. 1987,109, 1672. (6) (a) Kaim, W.; Kasack, V.;Binder, H.; Roth, E.; Jordanov, J. Angew. Chem., In?. Ed. Engl. 1988,27, 1174. (b) Ernst, S.;Kasack, V.;Kaim, W. Inorg. Chem. 1988, 27, 1146. (c) Kaim, W.; Kasack, V. Inorg. Chem. 1990, 29, 4696. (7) (a) Bruns, W.; Kaim, W. J . Organomet. Chem. 1990,390, C45. (b) Bruns, W.; Kaim, W. In ref IC, p 365. (c) Kaim, W.; Bruns, W.; Poppe, J.; Kasack, V. J . Mol. Struct. 1993, 292, 221. (d) Bruns, W.; Kaim,
W.; Ladwig, M.; Olbrich-Deussner, B.; Roth, T.; Schwederski, B. In Molecular Electrochemistry of Inorganic, Bioinorganic and Uganometallic Compounds;Pombeiro, A. J. L., McCleverty, J., Eds.; Kluwer: Dordrecht, The Netherlands, 1993; p 255. (e) Bruns, W. Ph.D. Thesis, University of Stuttgart, 1993.
0020-166919311332-2640$04.00/0
(NH,),Ru -N
K-3
w 1
2 Complexes 2n+ contain the unique bridging ligand 3,6-bis(2pyridyl)-1,2,4,54etrazine,which is distinguished by a low-lying T* orbital localized at the four tetrazine nitrogen atoms.s.9 The 4+ and 5+ states of 2 were described recently by Johnson, de Groff, and Ruminski.lo In contrast to the closely related bis(2,2’-bipyridine)rutheniumsystem (3”+),6b,9 the ammine complex 2n+was shown1° to form a mixed-valent 5+ state at rather low potentials with a very large comproportionationconstant Kc = log{[(E(4+/5+) - E(5+/6+)]/0.059 V) of 1014.2 in water, corresponding to AE = 840 mV. (The Creutz-Taube ion has AE = 0.390 mV and thus Kc = lO6.6.)2*3Despite this extremelystrong ’electrochemical” coupling between the metal centers in 2“+ as indicated by the Kc value, there was no indicationfor an IT band up to 1300 nm.10 We now report that this transition can be found, albeit in the form of an unexpectedly weak absorption ~~~
~
~
(8) (a) Kohlmann, S.;Ernst, S.; Kaim, W. Angew. Chem., In?. Ed. Engl. 1985,24,684. (b) Kaim, W.; Kohlmann, S.Inorg. Chem. 1987,26,68. (9) (a) Kaim, W.; Ernst, S.;Kohlmann, S.;Welkerling, P. Chem. Phys. Lett. 1985, 118, 431. (b) Jaradadt, Q.; Barqawi, K.; Akasheh, T. S. Inorg. Chim. Acta 1986, 116, 63. (c) Emst, S. D.; h i m , W. Inorg. Chem. 1989, 28, 1520. (d) Kaim, W.; Ernst, S.;Kasack, V. J . Am. Chem. Soc. 1990,112, 173. (10) Johnson, J. E. B.; deGroff, C.; Ruminski, R. R. Inorg. Chim. Acta 1991, 187, 73.
0 1993 American Chemical Society
Intervalence Transition in a Ru Complex
Inorganic Chemistry, Vol. 32, No. 12, 1993 2641
band, in the near-infrared (near-IR) region; we also present complete UV/vis and EPR spectroelectrochemical results for both of the persistent 5+ and 3+ states. The latter is of interest with respect to the role of the ruthenium coligands in comparison to those in 33+.
Table I. Physical Properties of Ligand-Bridged Ruthenium Dimers
B(6+/5+) E(5+/4+) E(4+/3+) E(3+/2+) Kc(3+) Kc(5+)
2 ElectrochemicalData‘ +0.87 (80) +1.58 (90) +0.44 (66) +0.69 (65) n.d. -0.75 (70) n.d. -1.54 (65) n.d. 1013.3 106.6 1015.0
LIT (
Absorption Datadfor the 5+ Ions 1453 (6880) 1560 (6400)
nS
h e
d
5000
500
1250
ca. 1600
3
Experimental Section Materials. Although complex 24+(PF6-)4 was obtained essentially according to the literature,1° the procedure had to be altered in two respects: The gram amount of bptz given in ref 10 is too high by 1 order of magnitude (0.149 mmol corresponds to 0.0353 g), and the volume of the reaction mixture had to be reduced 10-fold to 10 mL in order to ensure precipitation of 2‘+ as the analytically and spectroscopicallypure tetrakis(hexafluorophophate) salt. For vis/near-IR spectroscopicmeasurementsin DMSO and 1M DCl/ DzO, the mixed-valent ion 25+ was precipitated as a reddish purple salt with mixed bromide/hexafluorophosphate anions by treating the blue Ru(II)/Ru(II) precursor with bromine in ethanol. The identity of the oxidizing 5+ ion was confirmed by comparison with spectroelectrochemically obtained data (see Table I). Lnrhuwatation. EPR spectrawere recordedin theX band on a Bruker ESP 300 spectrometerequipped with a Bruker ER035M gaussmeterand an HP 5350B microwave counter. Cyclic voltammetry was carried out in dry acetonitrile/O.l M BuNPF6 using a three-electrodeconfiguration (GCE, Ag/AgCl, Pt) and a PAR 273/175 potentiostat and function generator. Absorption spectra were obtainedusing a Bruins Instruments Omega 10spcctrometer;the program Lab Calc (GalacticIndustriesCorp.) was used for spectra deconvolution. Spectroelectrochemistry. UV/vis/near-IR spectroelectrochemistry was performed in a previously described OTTLE (optically transparent thin-layer electrode) Electrolyses were carried out at preset potentials until no more spectral changes were observed. The stabilities of the oxidation states were checked by obtaining 100%regeneration of starting material upon reversed electrolysis and by the appearance of isosbestic points. The paramagnetic forms for EPR spectroscopy were generated electrolytically” or chemically, Le. using cobaltocene for the reduction of 24+.
EPR Datah 25+ 2.019 2.418 2.9 13 2.477
15+
gl g2
+2.02 (n.d.) +1.52 (60) -0.03 (60) -1.25 (irr) (ca. 1020) 108” n.d. n.d. n.d.
23+ 1.989 1.989 2.022 2.00
33+
1.346 2.489 n.d. g3 2.799 1.9980 (g) 2.298 From refs 2, 3, and 14. b From refs 9 and 10. C From cyclic voltammetry in acetonitrile solutions. Potentials in V vs SCE (1,3) or Ag/AgC1(2), the potential of SCE being slightly more positive (by 0.02 V) than that of Ag/AgCl. Peak potential differences (in mV) are given in parentheses. n.d. = not determined; irr = irreversible step (peak potentialgiven). Kc= 10exp(AE/0.059 V). Inacidicaqueoussolution. Absorptionmaximumin nm (andcm-l). Valuesfor Z5+ in othersolvents: 1433 nm (6980 cm-’, in CH,CN), 1463 nm (6840 cm-1, in DMSO). /Molar extinction coefficient at the absorption maximum in M-1 cm-1. E Bandwidth at half-height in cm-1. From measurementsat 3 K using a single crystal (15’) or a frozen acetonitrile solution (25+).
20 -
15
1
Q6 I
-
5
)
0
ea
1Mo
12w
14m
1Sm
lea
x
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Results
3b
1’
400 600
t 800 lo00 1200 1400 1600 1800
I. [nml The electrochemical results for the complex 24+(PF6-)4 in waterlo with two fully spectroelectrochemicallyreversible (3+/ 4+/5+) and two cyclovoltammetrically reversible steps (2+/ 3+, 5+/6+) could essentially be reproduced in acetonitrile, yielding the hE and Kcvalues summarized in Table I. Similarly, the main absorption features of the 4+ ion at 850 (sh), 599,and 367 nm and of the chemically and spectroelectrochemically generated 5+ ion a t 518 nm in acetonitrile are very similar to those reported for aqueous solutions;1° an additional maximum a t 347 nm was found for the 5+ form. Three isosbestic points occur a t 412, 556, and 1054 nm (Figure 1) for the (4+/5+) equilibrium. Our experimental facilities have now allowed us to detect two weak long-wavelength absorption bands of P.One broad band is rather solvent-sensitive, lying at 900nm in acetonitrile (e 1100 M-l cm-’), at about 800 nm in DMSO, and below 720 nm in 1 M DCl/D20. The other new band at about 1450 nm (Table I) is unsymmetrical (with some visible structuring in aqueous (11) Krejcik, M.;Danek, M.;Hartl, F. J. Electround. G e m . Interfacial Electrochem. 1991. 317, 179.
Figure 1. Vis/near-IR spectroelectrochemistryof Z4+++ in CH,CN/ 0.1 M BuNPF6at +0.95 Vvs Ag/AgCl. Theinsertshowsaneniargement of the near-IR region for Z5+ in 1 M DCl/D20 with the unsymmetrical,
slightly structured IT band. solution; Figure 1) and virtually solvent-sensitive with a molar extinction coefficient t of 500 M-l cm-l in CH3CN and 1 M DCl/D20 solutions (Figure 1). UV/vis spectroelectrochemistry was also performed for the (4+/3+) couple (Figure 2), yielding absorptions at 745, 559, and 393 nm for thesinglyreduced form. Spectroelectrochemistry beyond the 3+ and 5+ forms did not produce sufficientlypersistent species. EPR spectra of both the 3+ and 5+ paramagnetic forms could only be observed at low temperatures ( 10*3,~~~6990cm-1,e410M-1cm-1, A;1,2 1600 (Kc> 1083;rr 57 lOcm-l, cm-1),24((p-q2:q2-C6H6)[OS(NH~)S]}~+ e 220 M-l cm-l, A;1/2 1600 ~ m - l ) and , ~ ~bis chelate complexes of the type [(p-q2:q2-L)(RuL'n)23"(n = 3+ 6and 5+ (this work)).
Acknowledgment. Support from the Deutsche Forschungsgemeinschaft, Volkswagenstiftung, and Fonds der Chemischen Industrie is gratefully acknowledged. We also thank Dr. M. KrejEik (Prague, Czechoslovakia) for introduction to the spectroelectrochemical cell and Professor N. E. Katz (Tucuman, Argentina) for valuable suggestions concerning syntheses. ~~
(21) (a) Allen, G.C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (b) Hush, N. S.Prog. Inorg. Chem. 1%7,8,391. (c) Hush, N. S.Coord. Chem. Rev.1985, 64, 135. (d) Reimers, J. R.; Hush, N. S.In ref IC, p 29. (22) Kaim, W.; Moscherosch, M.; Kohlmann, S.; Field, J. S.;Fenske, D. J . Chem. Soc., Dalion Trans., in press. (23) Troll, T. Electrochim. Acta 1982, 27, 1311. (24) Tom, G.M.; Taube, H. J. Am. Chem. Soc. 1975, 97, 5310. (25) Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1987, 109, 1883.
